I've traveled to Europe in September or October almost every year since about 2016. Airfare, particularly United, this year is notably more expensive than any prior year. Many flights appear largely full in business and premium economy. Right now I could fly in PE in July or August for less than September or October which doesn't jive with the normal summer rush to Europe.

Is there something going on this year that has changed demand or supply sizeably?

Not that I know of. Olympics are late July, early August.

Depends on where you are going in Europe. Spain, Portugal, Italy and Greece have great weather and not so hot days in Sept and early October. Lots of travel to those destinations.

A co-worker mentioned the same the other day. He's looking to go to Germany in September and the fares are about $400 more than usual.

This is what I'm finding. Nothing under about $2k for PE unless I accept weird routings like an overnight at Heathrow on the way home. United is $3 to $5k for PE on some dates even though cabins are less than half full. I've paid $1500 to $2200 in prior years.

I've heard anecdotally that with the increasingly hot summers in Europe and black of AC that shoulder season is becoming more popular.

Airbus and Boeing are not delivering aircraft as fast as the airlines expected so there will not be the expected volume of new aircraft available. Without more seats, they'll increase the prices on the seats they have and, like another poster said, the gradual erosion of shoulder season means more people will be looking to travel instead of watch Mike Elko's Aggies shock the world to a natty this fall.